The present invention relates to a device for the introduction and mixing of a liquid fertilizer in a flow of water for irrigation and fertilizing the roots and foliage of plants. Principally, this device is for use by individuals. For fertilizing with an aqueous solution containing liquid fertilizer, it is known to introduce the required quantity of liquid fertilizer into the required quantity of water and to agitate it for mixing. In practice, the necessary quantity of water is introduced into a watering can, into a bucket or into a spray apparatus, then the user, with the help of a metering cap on the liquid fertilizer receptacle, which is usually present in the form of a bottle made of synthetic material, introduces the necessary quantity of liquid fertilizer. Such an operation gives rise to numerous problems. On the one hand, it requires manipulation of a relatively toxic liquid and does not avoid the risk of contact of this liquid with the skin, the mucous membranes or the eyes in case of splashing, which can be dangerous and on the other hand it does not avoid risk of error in the dosage which can lead to a lack of fertilizer or else to an excess which could destroy the plants.